Radio frequency identification systems are used in a wide number of applications. For example they are used as pass keys to open doors, gates and the like, they are used as product identification tags, they are used in toll collection systems and in similar situations where it is desirable to electronically read information at a distance and perform an act in response to the information read. In the past, such systems have generally been limited to relatively short range operation on the order of a few inches up to a few feet.
Long range use (on the order of up to or exceeding about 1020 feet) of RFID systems has been limited somewhat because there are a limited number of radio frequency bands available for such applications and some of those bands are subject to various types of radio frequency interference (RFI). Radio frequencies used for RFID applications typically include the ISM (Industrial-Scientific-Medical) frequency bands which are defined by international convention as well as the communications laws of individual countries. In the United States, the 902-928 MHz band and the 2400-2500 MHz band are commonly used as are other bands. Unlicensed use of these bands is subject to transmitted power and other technical limitations. Users of ISM bands must generally accept any interference present on those bands. One example of such interference is the broadband noise generated by CDMA (code division multiple access) spread spectrum mobile telephony in the 800-1000 MHz frequency range. This type of transmission tends to raise the noise floor in this and adjacent frequency bands because while the CDMA signals are not particularly strong on any particular frequency within the spread spectrum frequency band, the signal is constantly and rapidly changing frequencies within the spread spectrum frequency band and, as a result, the general noise level within the spread spectrum frequency band increases. Another example of such mobile telephony induced interference is the noise generated by TDMA (time division multiple access) mobile telephony in the same general range of frequencies. TDMA noise tends not to raise the noise floor, but rather to impart narrow bandwidth, relatively high amplitude spikes (and associated spurious signals) across the frequency band in which it is active, as well as in adjacent bands.
When designing RFID systems for use in vehicles (as in toll, gate access, and similar applications) as well as RFID systems for handheld use at a distance, relatively high-power CDMA and TDMA noise sources are now commonplace as individuals, both outside and particularly within the vehicles can often be expected to be using TDMA or CDMA cell phones and similar data communication devices which can potentially interfere with the use of such RFID systems.
What is desired is an RFID system operable at a distance up to or exceeding about 20 feet and which is not subject to CDMA, TDMA and similar types of interference and which includes a low power consumption component mountable in a vehicle for long term operation.